The present invention relates generally to apparatus for catalytic decomposition of monopropellant fuels and more specifically to a plurality of stacked thin metal plates having precise flow passages to provide selected uniform flow characteristics across each plate while promoting thorough catalytic surface contact with well mixed monopropellant.
Catalytic decomposition of monopropellant fuels, i.e., H2O2 and N2H4, requires the use of a transitional metal catalyst to initiate and sustain decomposition. Catalyst beds are designed to supply large surface areas of catalytic substance and thorough mixing of the monopropellant to facilitate complete decomposition. A peroxide catalyst bed typically uses silver screen packs, while a N2H4 system typically consists of iridium deposited on alumina granules. Extruded ceramic cores have also been employed.
In all instances the catalyst was applied to an aggregate material. These materials are subject to wear and are relatively fragile (the alumina granules), and do not possess a consistent flow resistance (stacked screens). The uneven flow resistance leads to localized flow restrictions which result in recirculation of the decomposed flow, unpredictable start/stop behavior and unpredictable pressure drop through the pack. The extruded ceramic cores provide straight through passages that do not promote mixing of the monopropellant, thus limiting the catalytic surface contact.
Examples of prior art catalyst beds are found in issued U.S. Pat. Nos. 3,535,879; 4,211,072; 4,517,798; 4,856,271; 4,938,932; and 5,531,968. A further example of a conventional catalyst bed is discussed below in conjunction with FIG. 4 of the accompanying drawings.
The invention provides a bonded stack of very thin metal plates, referred to as platelets, that provide very high surface area per unit of volume and precise flow passages or holes in a given cross section, resulting in high, stable and repeatable performance. Because the flow passages are precisely photo etched, the flow restriction and therefore the flow rate is uniform across each platelet. Also, by varying the diameter of the etched holes, the fraction of the cross section that is open on a given platelet can be precisely designed to give the fluid the desired pressure drop and volume to expand.
The platelets are segregated into distinct groups separated from one another by a metering platelet. Each metering platelet has slightly smaller flow passages than the groups preceding and following. This xe2x80x9crestrictionxe2x80x9d will inhibit the formation of hot spots by recirculation due to non-uniform flow in an upstream group. Good fluid mixing is promoted by offsetting passages from platelet to platelet. Larger and less frequent flow passages may be used instead of smaller flow passages in the metering platelet as long as the total flow-through area is less than the platelets in the adjacent groups.
It is therefore a principal object of the present invention to provide a catalyst bed comprising a stacked plurality of contiguous thin metal plates having flow-through holes of selected size and at selected locations to promote uniform flow of a fluid through the bed.
It is another object of the present invention to provide a catalyst bed having a generally cylindrical configuration and designed to promote uniform mixing and efficient catalyst contact of a fluid flowing axially through the bed.
It is still another object of the invention to provide a catalyst bed formed from a stacked array of thin metal plates having catalyst material surfaces and axial flow-through holes and being segregated into a plurality of groups of such plates, each such group being separated from adjacent groups by a metering plate having a smaller flow area.